1. Field of the Invention
This invention relates to method and apparatus for linearizing the transfer characteristics of a wideband, temperature stable amplifier and, more particularly, to reducing the distortion encountered in a broadband silicon based bipolar junction transistor (BJT) amplifiers, CATV repeaters, and electro-optic driver amplifiers.
2. Description of the Prior Art
Predistortion and linearization methods and apparatus are well known in the art of CATV systems to compensate for distortion of amplified broadband signals. Most of the known methods are complex and have high insertion loss in order to provide substantial magnitude of distortion cancellation ( greater than 10 dB, typically). Some methods utilize a closed loop approach; while, other methods are open loop systems. Many of the prior art linearization methods encounter high insertion loss (several dB) and require additional amplification stages for implementation.
Most of the prior art methods also have a limited range of output level where best results occur and they are restricted to near room temperature operation. Furthermore, their adjustment must be tailored to a specific channel plan frequency assignment set. For CATV outdoor application, the linearization method must be operable over a wide temperature range. For logistical reasons, it must be capable of good performance over a wide range of operating levels and channel plans. Therefore, there is need for a linearization method directed to low insertion loss, for example, 2 dB or less, and characterized by very low power consumption.
Many of the prior art methods utilize diode-based circuits to reduce composite triple beat (CTB) and cross modulation (XM) distortion. Most methods use complex circuitry and normally experience high insertion loss (up to 10 dB). To accommodate such a large insertion loss, systems that use these methods require additional amplification stages to compensate for the loss. As a result, most linearization methods have been limited to complex, high cost circuitry usable only in headend equipment, such as 1310 nm and 1550 nm fiber optic forward path transmitters.
U.S. Pat. No. 5,703,530 discloses a radio-frequency amplifier circuit that utilizes a distortion generation circuit (xe2x80x9cDGCxe2x80x9d). The DGC is employed in the radio-frequency amplifier to reduce composite triple beat distortion and cross modulation distortion and to linearize the gain across the desired bandwidth. The radio-frequency amplifier is disclosed with the DGC inserted at selected points in the amplification stage. For example, the DGC is positioned in a number of embodiments at the input, output, both the input and output and intermittently between the input and the output. The ""530 patent approach was mainly intended for inclusion within an RF hybrid, as opposed to external to the RF hybrid, as the present invention. Furthermore, the ""530 patent does not address the need for temperature compensation, nor the wide range of operating levels and channel plans needed for CATV repeater use.
U.S. Pat. No. 5,600,472 discloses a distortion generating circuit that includes a low-distortion amplifier for giving a nonlinear distortion component to an input signal and an amplifier for receiving a signal output from the distortion generating circuit and amplifying it. The distortion generating circuit is not suitable for general purpose outdoor use with equipment providing multiple channel, fixed gain repeaters for analog transmission, such as CATV RF amplifiers.
U.S. Pat. No. 5,939,920 discloses an apparatus and method for providing distortion to an input signal.
Other relevant prior art includes U.S. Pat. No. 5,523,716 which discloses a microwave predistortion linearizer. This is not relevant to CATV repeater amplifier application. As with most of the above-discussed prior art patents, the prior art devices do not address the need for temperature compensation in predistortion applications.
Therefore, there is a need in predistortion applications for simple, low cost, and low loss technique, together with temperature compensation to control the nonlinear element bias current to compensate for known variations in forward diode voltage and amplifier distortion level changes with temperature. This would allow a transfer linearization technique to be used in outdoor applications subject to a wide temperature variation and a variety of operating levels and channel loading at a low cost, with high reliability, and good distortion reduction.
In accordance with the present invention there is provided a method for reducing second and third order distortion in RF amplifiers and broadband transmission systems caused by amplifier compression in silicon-based bipolar RF amplifiers. The technique also extends to driving electro-optic devices, such as laser transmitters, which exhibit compressive transfer characteristics at high signal levels. A first ac current component is separated from the input signal to provide a first signal representing the first alternating current component and a second signal representing the input signal having the first ac component is separated therefrom. Nonlinear attenuation to the second signal is provided to produce a modified second signal. The modified second signal is combined with the delayed and level adjusted first signal to produce a corresponding output signal.
Further in accordance with the present invention, there is provided a method for supplying a modified RF signal to an amplifier. A first portion of an unmodified RF signal is passed through a linear attenuator. Then the first portion of the unmodified RF signal is passed through a time delay component to form a first portion of a modified RF signal. A second portion of an unmodified RF signal is passed through a dc blocking device to a nonlinear attenuator using one or more biased diodes. The second portion of the unmodified RF signal is modified by the nonlinear attenuator to form a second portion of a modified RF signal. The first portion of a modified RF signal is combined with the second portion of a modified RF signal to form a modified RF signal for supplying to an amplifier. The modification incorporated in the signals complements the amplitude and time delay transfer characteristic of the amplifier, thereby reducing distortion.
Accordingly, a principal object of the present invention is to incorporate the transfer linearization circuit in a bridged tee network. The bridged tee network is characterized by constant impedance, allowing amplitude equalization versus frequency.
The present invention provides for a wideband, low insertion loss, low cost, high reliability third order distortion reduction of 4 to 10 dB over a wide temperature range using simple temperature compensation of the nonlinear circuit element(s).
Another object of the present invention is to provide a transfer linearization circuit for reducing distortion in CATV systems in which independently biased diodes provide simultaneous reduction of both second and third order distortions.
A further object of the present invention is to provide in a transfer linearization circuit with low insertion loss, and at high frequencies to provide loss compensation, so the overall frequency response is flat while maintaining a constant impedance.
An additional object of the present invention is to provide in CATV systems utilizing RF amplifiers wideband distortion reduction both in-band and out-of-band, further, to provide good distortion reduction for a variety of operating levels and channel plans over a wide temperature range.